Increasing applications for holographic optical elements have resulted in continued development of new effective and reliable photosensitive media. P. Hariharan in his book pages 95 and 96 [herein after noted as Ref. 1] reports that the main photosensitive materials available for high efficiency hologram recordings are silver halide photographic emulsions, dichromated gelatin, photoresists, photopolymers, photothermoplastics, polymers with spectral hole-burning, and photorefractive crystals. Each of these materials has their merits, but all have drawbacks. These organic materials (photographic emulsions, dichromated gelatin, and photopolymers) are sensitive to humidity. Moreover, they significantly shrink in the development process. Inorganic materials (photorefractive crystals) have low resistance to elevated temperatures and produce additional patterns because of exposure to the beam diffracted from the recorded grating.
The ideal recording material for holography should have a spectral sensitivity well matched to available laser wavelengths, a linear transfer characteristic, high resolution, and low noise, be indefinitely recyclable or relatively inexpensive. Hariharan reports on page 95 of his book [ Ref. 1] (see listing of references before claims section of this invention) that xe2x80x9cWhile several materials have been studied, none has been found so far that meets all these requirementsxe2x80x9d. The lack of available materials for phase holographs has stimulated the search for new approaches.
A photo-thermal process based on precipitation of dielectric microcrystals in the bulk of glass exposed to UV radiation was reported by Stookey [hereinafter referred to as Ref. 2] Stookey""s two-step process (exposure and thermal development) was used to record a translucent image in glass because of light scattering caused by a difference between refractive indices of a precipitated crystalline phase and the glass matrix. Later, colored images were recorded in similar glasses by photo-thermal precipitation of a number of complex crystals of different compositions, sizes and shapes. According to these studies, the first step is the exposure of the glass sample to UV radiation, which produces ionization of a cerium ion. The electrons released from cerium are then trapped by a silver ion. As a result, silver is converted from a positive ion to a neutral atom. This stage corresponds to a latent image formation and no significant coloration occurs.
The next step is a thermal development. This development process includes two stages described in publications [hereinafter referred to as Ref. 3, and hereinafter referred to as Ref. 4]. The first involves the high diffusion rate silver atoms possess in silicate glasses. This diffusion leads to creation of tiny silver crystals at relatively low temperatures (450-500xc2x0 C.). A number of silver clusters arise in exposed regions of glass after aging at elevated temperatures. These silver particles serve as the nucleation centers for sodium and fluorine ion precipitation and cubic sodium fluoride crystal growth occursfat temperatures between 500xc2x0 C. and 550xc2x0 C. Further heat treatment leads to growth of numerous elongated pyramidal complexes of (Na,Agxe2x80x94F,Br) crystals on the surface of cubic NaF crystals. This mixture of crystals can produce opal coloration in the case of large crystal sizes or yellow coloration caused by colloidal silver precipitated on interfaces of dielectric crystals. A second exposure to UV followed by a second heat treatment produces different coloration because of metallic silver reduction on the surfaces of the dielectric pyramids. The final resulting coloration depends on the size and aspect ratio of these silver particles. This multi-stage photo-thermal process in photosensitive glass was proposed for decoration, color photography, sculpture, and even for holography (See [Ref. 3, Ref. 4 and Ref. 5]). However no evidences of any hologram recorded in these glasses are in those references.
Several years later, the use of inorganic photosensitive glasses for phase hologram recording rather than as a photographic medium was reported in the literature: Bragg gratings were obtained both in lithium-aluminum-silicate (Ref. [6]); and sodium-zinc-aluminum-silicate (Ref. [7] and Ref. [8]); glasses doped with silver and cerium; by exposure to UV radiation followed by thermal treatment. This phenomenon was named the xe2x80x9cphoto-thermo-refractivexe2x80x9d (PTR) process. Glasses, which possess such properties, were called xe2x80x9cphoto-thermo-refractivexe2x80x9d (PTR) glasses.
It was reported in Refs. [7] and [8] that a refractive index decrease of about 5xc3x9710xe2x88x924 occurs when dielectric crystals precipitated in glasses are exposed to radiation of a nitrogen laser at 337 nm. The refractive index of NaF in the red spectral region is nNaF=1.32 whereas the refractive index of PTR glass nPTR=1.49. The small value of refractive index change is due to the small volume fraction of precipitated crystalline phase; however, it is sufficient to result in high efficiency Bragg grating recording in samples with thickness more than several hundreds of microns. Conditions of glass exposure and development were found in that work to create Bragg gratings with relative diffraction efficiency up to 90% and angular selectivity up to 2 mrad. The maximum recorded spatial frequency was 2500 mmxe2x88x921. These gratings were stable up to 400xc2x0 C. The photosensitivity was found in the range of several J/cm2 at a nitrogen laser wavelength (337 nm). The absorption band of Ce3+, which is used for photo-ionization, has maximum near 300 nm and a long wavelength tale up to 400 nm. This means that several commercial lasers such as N2, Ar, Hexe2x80x94Cd, etc., can be used for recording. Once developed, holograms in PTR glass were not destroyed by further exposure to visible light.
Unfortunately, these materials reported in Refs. [6-8] do not met all requirements formulated in Hariharan, particularly absolute diffraction efficiency [1, Table 7.1 at page 96] because their property of excessive (strong) scattering of the radiation imposed on the hologram. This scattering results in low absolute diffraction efficiency of gratings in PTR glasses, which has been found not to exceed 45%. Thus, this PTR material for holographic optical elements described in Refs. [6-8] had serious drawbacks; particularly, inadequate absolute diffraction efficiency which results in excessive scattering of the radiation.
The first objective of the present invention is to provide a method for preparing a holographic element having an absolute diffraction efficiency of greater than 50 percent.
The second objective of this invention is to provide a holographic element from PTR glass having an absolute diffraction efficiency of greater than 90 percent.
The third objective of this invention is to provide a holographic element from PTR glass having an absolute diffraction efficiency of at least 93 percent.
The fourth objective of this invention is to provide a procedure of photo-thermo-refractive glass fabrication which provide a high absolute diffraction efficiency of recorded Bragg gratings.
A preferred embodiment of the invention is a process comprising the steps of: fabrication of a photo-thermo-refractive (PTR) glass containing oxides of sodium, zinc, aluminum, and silicon as main components, fluorine, bromine, tin, antimony, fluorine, and bromine as dopants, and no impurities of iron and heavy metals in concentrations more than 5 parts per million, exposing said glass to patterned ultraviolet radiation in spectral region from 280 to 400 nm for dose above 50 mJ/cm2, thermal development at temperatures in the range from 480 C. to 580 C. whereby the resulting Bragg grating has an absolute diffraction efficiency of at least 90%.
Further objectives and advantages of this invention will be apparent from the following detailed description of the presently preferred embodiments which are disclosed in the following text and properties of which are illustrated in the accompanying drawings.